When I bought my Sigma 40VE used from a gun shop the first thing I noticed when I took it to the range was that the rear sights were pushed all the way to the left (I think left, don't really remember). I'm thinking the previous owner had the same problem.

I put a laser bore sighter in it to line the sights back up and had no problem with being on target.

Yup - trigger control.

All the above advice is very good but I have a suggestion that helped me improve my trigger control overnight, install a laser sight and practice dry firing. If you can press the trigger without that dot moving you will have better groupings on target, it's a great tool to learn and maintain trigger control when used with the good advice already given here.